Retrieving an instance using a string literal with Google Guice

Question

I have multiple modules with service interfaces binding to their corresponding types and I am able to get an instance by using

injector.getInstance(MyServiceInterface.class) 

I would like to retrieve the instance using

injector.getInstance("MyServiceInterface")

ie a string literal instead of the class type

How can I achieve this ?

To elaborate my question further - I can retrieve the Class object from the string literal using a Class.forName(literal) call and then use it to retrieve the instance with a injector.getInstance(clsInstance) .

After retrieving the instance which I receive in my base service type interface I need to use reflection to invoke the method of the service object.

so Service serv = injector.getInstance(MyCustomService.class)

Now I need to invoke myCustomMethod() present in MyCustomService through reflection since this invoker is generic and is intended to work with multiple services without being aware of their actual type.

I will also need the Method interceptors configured on the service interfaces to be invoked transparently when I invoke the method on this instance reflectively.

Answer 1

While I'm not certain if there's functionality for that built into Guice itself, you could try getting the relevant Class<?> object yourself.

Something along the lines of:

Class<?> myServiceInterfaceClass = Class.forName("path.to.MyServiceInterface");
injector.getInstance(myServiceInterfaceClass);

This does however require that the current Classloader can access that specific class, etc.

Answer 2

This can't be done within Guice... because it can't be done, period! Think about it, let's say you have two of the same class name in different packages . Which class would you instantiate?

So at the very least the String would have to have the fully qualified class name, eg instead of Integer , it would have java.lang.Integer .

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However, if you know which classes you want to support in advance, you can use a MapBinder.

Tweaking their example to match your use case:

public class ServiceModule extends AbstractModule {
    protected void configure() {
        MapBinder<String, MyServiceInterface> mapbinder
                = MapBinder.newMapBinder(binder(), String.class, MyServiceInterface.class);
        mapbinder.addBinding("MyServiceInterface").to(MyServiceImpl.class);

        bind(MyServiceInterface.class).to(MyServiceImpl.class);
   }
}

Now you can inject like this:

class ServiceManager {
    @Inject
    public ServiceManager(Map<String, MyServiceInterface> services) {
        MyServiceInterface service = stacks.get("MyServiceInterface");
        // etc.
    }
}

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Please note when you call inj.getInstance() you do have to know the return type of the Object you're trying to create, unless you are planning on doing:

Object foo = inj.getInstance(myString);